docs: def _make_axes_dict(self, axes):
          """Makes an axes dictionary.
  
          .. note::
  
              In case the input is ``None``, the dictionary :code:`{'empty': None}`
              is returned.
  
          **Function-call argument** \n
  
          :param axes:    axes input
          :type axes:     dict or single instance of
                          :class:`~climlab.domain.axis.Axis` object or ``None``
          :raises: :exc:`ValueError`  if input is not an instance of Axis class
                                      or a dictionary of Axis objetcs
          :returns: dictionary of input axes
          :rtype: dict
  
          """
          if type(axes) is dict:
              axdict = axes
          elif type(axes) is Axis:
              ax = axes
              axdict = {ax.axis_type: ax}
          elif axes is None:
              axdict = {'empty': None}
          else:
              raise ValueError('axes needs to be Axis object or dictionary of Axis object')
          return axdict